Work piece flaw detection and classification systems are known. The Republic Steel Corporation has developed a number of innovative flaw detecting systems and a number of patents have issued describing these systems.
One of the earlier Republic Steel flaw detecting patents is U.S. Pat. No. 2,660,704 to Harmon. That patent discloses a test device for locating flaws such as cracks, seams, breaks, and the like in a steel workpiece by measurements conducted at the surface of the workpiece. The apparatus disclosed in the '704 patent includes a search unit adapted to be positioned upon or adjacent a workpiece and for subjecting the workpiece to a periodically varying electro-magnetic field produced by a coil carried in the search unit. Suitable control and flaw indication circuitry is coupled to the search unit to convert signal variations produced by movement of the search unit about the workpiece circumference into an indication as to the relative characteristics of the various portions of the bar. U.S. Pat. Nos. 2,914,726 and 2,832,040 disclose improvements and refinements in the technique disclosed in the '704 patent.
Two more recent Republic Steel patents disclose control circuitry and apparatus for utilizing the detecting principle disclosed in the '704 patent to classify and mark the position of defects in steel bars as they are tested. These two patents are U.S. Pat. Nos. 3,108,230 to Judd et al and 3,263,809 to Mandula et al. Both patents are used in conjunction with a test probe which is moved relative to a steel bar and coupled to energization circuitry for generating electromagnetic signals within the bar.
According to the '809 patent, the defect information obtained by the probe or test head is acted upon by a detection circuit which produces an output voltage pulse in response to the presence of a defect. The pulse amplitude is related to the depth of the defect and is introduced to a classifier circuit having two channels. One channel generates a trigger signal each time a relatively deep defect is sensed and a second channel generates a trigger pulse each time either a shallow or a deep defect has been sensed. These trigger pulses are introduced into an analyzer section which counts the number of shallow and deep defect trigger pulses for a given area of the workpiece, and from this information determines the severity of the combinations of defects. The analyzer section then classifies the workpiece or bar according to the combined severity as either good, salvage, or scrap.
As the name suggests, "good" workpieces are those which have no defects, or defects which are not objectionable since they do not impair the utility of the product for its intended purpose. Workpieces classified as "salvage" have defects deep enough to be objectionable but not too severe to preclude repair. A "scrap" workpiece is one in which the defects are so deep and so long that it is not possible to salvage the workpiece.
The '230 patent discloses defect marking apparatus which marks the location of the defects on the workpiece. The marks made are permanent and easily visible even though the workpiece may be subject to abrasive handling. The '230 patent discloses a rotating cutter of carbide or other material which is movable to engage the workpiece and cut impressions therein at defect locations. An actuator is adapted to move the cutter onto the workpiece for the duration of an energizing pulse. Actuation is coordinated with defect detection equipment constructed, for example, in accordance with the earlier Harmon patent. The detection equipment and workpiece are relatively rotatable such that the detection equipment describes a helical path around a longitudinally moving workpiece. The control circuit is connected to both the detection equipment and to the actuator on the defect marking equipment and causes an energizing signal to activate the actuator to effect appropriate defect marking.
Since the cutter or marking system and the defect detection equipment cannot be at the same physical location, the marker is placed behind or "downstream" of the detection equipment. Both are preferably placed on a longitudinal line along the bar. The longitudinal space between the marker and the detection equipment is chosen to equal the distance of travel per revolution of workpiece movement. Upon receipt of a defect signal from the detection equipment, the control circuit coupled to the marker system delays sending an energization signal for a period of time equal to one bar revolution. Since forward bar travel per revolution is equal to the spacing between the detection equipment and the marker, the energization signal causes the mark to be applied to the bar at the location of the detected defect.
The concepts embodied in the Mandula et al and Judd et al patents enable bars to be marked and classified in a unified system which has proven to be very effective. The control circuitry utilized, however, for coordinating bar marking and classification is an analog electronic circuit which is not always sensitive to closely spaced multiple defects. The prior classification and marking system can only classify seams or cracks once every revolution of a bar or workpiece. If a plurality of deep seams exist about the circumference of the bar, the Mandula et al system analyzes only the first such defect and ignores subsequently sensed defects. This lack of sensitivity in defect sensing can result in errors in bar classification. If a short deep defect is first sensed along the path of workpiece travel, an accompanying long deep defect may not be properly measured and therefore a workpiece classified as good may in reality be either a salvage or scrap bar.
The prior art classification and marking system includes a plurality of separate analog timing circuits for controlling and coordinating classification and marking. Both the deep and shallow flaw measuring channels in the classification circuit each has its own timing mechanism which resets the classification circuitry in its associated channel after each bar revolution. In addition to these two timing circuits a third timing circuit is required to delay the marking of flaw position on the workpiece. Typically, more than one marker is positioned along the workpiece and these multiple markers each require a separate timer. As bars of different physical dimensions are examined, each of the multiple timers must be adjusted by the system operator.
In the prior art classifying process an attempt was made to resynchronize the multiple timing control circuits in response to the presence of flaws along the bar. The resynchronization circuitry was analog electronics, however, and was subject to drift in operating characteristics with temperature. Synchronization between the timing circuitry and bar rotation could be lost with possible inaccurate bar classification as well as inaccurate marking of flaw location.
Thus, although the prior art techniques for marking and classifying steel bars were effective, they exhibited shortcomings. In particular, the resynchronization and timing procedures utilized in the prior art classifying system were somewhat ineffective. In addition to these disadvantages, the prior art analog circuitry was weighty, consumed a moderate amount of power and was subject to drift in operating characteristics with temperature.